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Saturday, 1 February 2020

In a binary system, a rapidly rotating white dwarf carries space-time with it

From black holes to gravitational waves to the effects of gravitational lenses, Albert Einstein's theory of general relativity has made many predictions that have been validated experimentally over time. However, some of these predictions are less known than others. This is the case of the Lense-Thirring effect, which describes a phenomenon of space-time training around very dense objects in very fast rotation. Launched in 2004, NASA's Gravity Probe B satellite made it possible to confirm this phenomenon experimentally in 2011. And recently, astrophysicists have once again been able to confirm the Lense-Thirring effect around a white dwarf as part of a binary system.

In everyday life, this phenomenon is both undetectable and inconsequential, because the effect is ridiculously small. Detecting the spacetime entrainment caused by Earth's rotation requires satellites such as the $ 750 million Gravity probe B and detecting angular changes in gyroscopes equivalent to one degree every 100 '000 years approximately.

Fortunately, the Universe contains many natural gravitational laboratories where physicists can observe Einstein's predictions in detail. This new study, published in the journal Science , reveals evidence of the Lense-Thirring effect on a much more noticeable scale, using a radio telescope and a unique pair of compact stars rotating around each other. on the other at dizzying speeds.

A white dwarf and a pulsar to confirm the Lense-Thirring effect

The movement of these stars would have made astronomers perplexed at Newton's time, because they move in a distorted space-time, and require the general theory of relativity of Einstein to explain their trajectories. One of his least known predictions of this theory is that rotating bodies carry with them space-time. The faster an object turns and the more massive it is, the more powerful the drive.

White dwarfs are a great place to study this process. They are similar in size to Earth but hundreds of thousands of times more massive. They can also rotate very quickly, up to one revolution per minute. The training caused by such a white dwarf would be about 100 million times more powerful than that of Earth.

Illustration showing the Lense-Thirring effect as part of a white-pulsar dwarf binary system. Credits: Mark Myers / OzGrav ARC Center of Excellence

Twenty years ago, the CSIRO's Parkes radio telescope discovered a unique star pair made up of a white dwarf (the size of Earth but about 300,000 times more massive) and a radio-pulsar. Pulsars are made up of closely related neutrons , which makes them incredibly dense. In addition, they spin much faster than white dwarfs: 150 revolutions / minute for the pulsar studied by the authors.

PSR J1141-6545: an ideal gravitational laboratory for studying general relativity

This means that, 150 times per minute, a beam of radio waves emitted by this pulsar scans our point of observation here on Earth. Astrophysicists can use it to map the trajectory of the pulsar as it orbits the white dwarf, timing when its pulse arrives at the telescope, and knowing the speed of light. This method revealed that the two stars orbit each other in less than 5 hours.

This pair, officially called PSR J1141-6545, is an ideal gravitational laboratory. Since 2001, researchers have used Parkes several times a year to map the orbit of this system, which has a multitude of gravitational effects.

Although PSR J1141-6545 is several hundred quadrillion kilometers (one quadrillion represents a million billion), the data shows that the pulsar rotates 2.54 times per second, and that its orbit varies in space. This means that the plane of its orbit is not fixed, but rotates slowly.

Binary system: the companion star accelerates the rotation of the white dwarf

When pairs of stars are born, the most massive one dies first, often creating a white dwarf. Before the second star dies, it transfers matter to its white dwarf companion. A disc forms when this material falls towards the white dwarf, and over tens of thousands of years, it accelerates the latter, until it makes a complete revolution every few minutes.

Many binary systems involve a giant star and a white dwarf, the latter accreting matter from the former. This accretion leads to an acceleration of the rotation of the white dwarf. Credit: Pearson Ed

In rare cases like this, the second star can then explode as a supernova, leaving behind a pulsar. The rapidly spinning white dwarf carries space-time with it, rocking the pulsar's orbital plane as it moves. This inclination is what astrophysicists have observed through the mapping of the orbit of the pulsar.


Lense–Thirring frame dragging induced by a fast-rotating white dwarf in a binary pulsar system

V. Venkatraman Krishnan, M. Bailes, W. van Straten, N. Wex, P. C. C. Freire, E. F. Keane, T. M. Tauris, P. A. Rosado, N. D. R. Bhat, C. Flynn, A. Jameson1, S. Osłowski

Science  31 Jan 2020:

Vol. 367, Issue 6477, pp. 577-580

DOI: 10.1126/science.aax7007

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